Fuel cell systems are seen as a promising alternative to traditional power generation technologies due to their low emissions, high efficiency and ease of operation. Fuel cells operate to convert chemical energy into electrical energy. Proton exchange membrane fuel cells comprise an anode, a cathode, and a selective electrolytic membrane disposed between the two electrodes. In a catalyzed reaction, a fuel such as hydrogen, is oxidized at the anode to form cations (protons) and electrons. The ion exchange membrane facilitates the migration of protons from the anode to the cathode. The electrons cannot pass through the membrane and are forced to flow through an external circuit thus providing an electrical current. At the cathode, oxygen reacts at the catalyst layer, with electrons returned from the electrical circuit, to form anions. The anions formed at the cathode react with the protons that have crossed the membrane to form liquid water as the reaction product.
Proton exchange membranes require a wet surface to facilitate the conduction of protons from the anode to the cathode, and otherwise to maintain the membranes electrically conductive. It has been suggested that each proton that moves through the membrane drags at least two or three water molecules with it (U.S. Pat. No. 5,996,976). U.S. Pat. No. 5,786,104 describes in qualitative terms a mechanism termed “water pumping”, involving the transport of cations (protons) with water molecules through the membrane. As the current density increases, the number of water molecules moved through the membrane also increases. Eventually the flux of water being pulled through the membrane by the proton flux exceeds the rate at which water is replenished by diffusion. At this point the membrane begins to dry out, at least on the anode side, and its internal resistance increases. It will be appreciated that this mechanism drives water to the cathode side, and additionally the water created by reaction is formed at the cathode side. Nonetheless, it is possible for the flow of gas across the cathode side to be sufficient to remove this water, resulting in drying out on the cathode side as well. To maintain membrane conductivity, the surface of the membrane must remain moist at all times. Therefore, to ensure adequate efficiency, the process gases must be, on entering the fuel cell, at an appropriate humidity and at a suitable temperature for keeping the membrane moist. The range for suitable humidities and temperatures will depend on system requirements.
A further consideration is that there is an increasing interest in using fuel cells in transport and like applications, e.g. as the basic power source for cars, buses and even larger vehicles. Automotive applications are quite different from many stationary applications. For example in stationary applications, fuel cell stacks are commonly used as an electrical power source and are simply expected to run at a relatively constant power level for an extended period of time. In contrast, in an automotive environment, the actual power required from the fuel cell stack can vary widely. Additionally, the fuel cell stack supply unit is expected to respond rapidly to changes in power demand, whether these be demands for increased or reduced power, while maintaining high efficiencies. Further, for automotive applications, a fuel cell power unit is expected to operate under an extreme range of ambient temperature and humidity conditions.
All of these requirement are exceedingly demanding and make it difficult to ensure a fuel cell stack will operate efficiently under all of the possible range of operating conditions. Meeting these requirements depends on maintaining effective control over the fuel cell operating conditions.
Accordingly, there remains a need for a fuel cell gas management system that can offer rapid dynamic control of fuel cell operating conditions.